Comparative virulence of seasonal viruses responsible for lower respiratory tract infections: a southern European multi-centre cohort study of hospital admissions

Objectives Our study aimed to compare the clinical severity of lower respiratory tract infections (LRTI’s) caused by Influenza and Respiratory Syncytial Virus (RSV). Methods We conducted a retrospective cohort study of LRTI admissions with positive PCR results for Influenza or RSV from 2017 to 2019 in three teaching hospitals in southern Europe. Data on clinical characteristics, viral agents and disease outcome were collected. Nosocomial infection was excluded. Main outcomes were invasive mechanical ventilation and in-hospital death. Results A total of 984 patients were included. Median age was 75 years. Influenza A was the most frequently identified virus (56.5%), of which 27.1% were subtype H1N1 and 53.0% H3N2. Influenza B was isolated in 22.3% and RSV in 21.0%. There were 10.5% of patients who died during admission and 8.3% submitted to IMV. Influenza A H1N1 was associated with lower age and less co-morbidity, while the opposite was observed for RSV. Influenza A H1N1 was independently associated with both higher risk of death (adjusted odds ratio 2.0 [1.2–3.4] p = 0.008) and IMV (adjusted odds ratio 5.1 [3.0–8.5] p < 0.001). Conclusion Influenza A H1N1 was an independent predictor of mortality and IMV. These findings may have implications on hospital resource planning and vaccination policies.